In the automobile and aircraft industries there is increasing use of components in which plastic workpieces have to be positively connected to other workpieces made of metal or composite materials. Methods which utilize the deformability of the plastic workpiece are particularly suitable for this.
The prior art discloses a series of methods with the aid of which a plastic workpiece is connected to a further workpiece by utilizing the deformability of the plastic workpiece. For example, it is known that a plastic workpiece has a projection that extends away from an abutting surface. On the abutting surface there is placed a further workpiece, which has a through-opening. This through-opening is formed in such a way that it receives the projection, the projection protruding out of the further workpiece through the through-opening. To connect the plastic workpiece to the further workpiece, the part of the projection that is protruding out of the further workpiece is mechanically deformed. In the simplest embodiment of this method, so much pressure is exerted on the projection by means of a tool that the projection is plastically deformed. The plastic workpiece and the further workpiece are said to have been cold connected. This connecting method is known as staking and has very short cycle times.
In many plastics, however, cracks occur in the projection during such cold connecting, having the effect of restricting the durability of the connection between the further workpiece and the plastic workpiece. In order to reduce the loading caused by the deformation, this projection may be brought into a plastic state before the deforming. This may be performed, for example, by blowing in hot air. In this case, however, the plastic is heated only very slowly by the hot air, greatly increasing the cycle time of the method. In other embodiments, a heated tool is used for deforming the projection. In this case, the tool is heated by means of heating resistors, ultrasound, infrared radiation, laser radiation or other means, depending on the embodiment. The projection brought into a plastic state can be plastically deformed without the material being subjected to great loads. The “warm” connections created in this way are much more durable than the previously mentioned “cold” connections.
However, all methods that provide a warm connection have a particularly high energy consumption, since the tools have to be very hot to achieve a short cycle time. The ultrasonic and laser heaters that are regularly used for this are also extremely complex to produce and maintain.
Finally, it is known from U.S. Pat. No. 7,497,917 that, when connecting a plastic workpiece to a metallic workpiece, a friction tool is brought into contact with the region of the surface of the metallic workpiece that surrounds the opening through which the projection of the plastic workpiece extends. The rotating friction tool introduces energy into the metallic workpiece, which leads to heating thereof and subsequently also heating of the plastic workpiece and the projection provided on it. As a result, the projection is brought into a plastic state, and a positive connection is obtained.
A disadvantage here is once again that the cycle times are comparatively long on account of the required heat transfer between the metal workpiece and the plastic workpiece, the heat transfer being influenced decisively by the nature of the abutting surface between the plastic workpiece and the metal workpiece.